A Guide to Engineered Wood Sheathing for Inspectors and DIY-ers

The use of engineered wood sheathing
has grown dramatically in residential construction over the past 30 years. Engineered
wood products are often used for
sheathing in residential construction on roofs, floors and walls.

What Is Engineered Wood?

Technically, engineered wood is any
wood product that is not solid wood. Producers of wood panel products like to
call plywood the first engineered product. Plywood consists of thin veneers of
wood that are stacked so that the grain of one layer, or ply, runs
perpendicular to the plies above and below it. An adhesive is applied between the
plies and they’re bonded together on a hot press.

In the early 1980s, producers created
technology to take the plywood concept a step further. Rather than bond
together thin veneers of wood, the new process allowed for the bonding together
of small pieces, or strands, of wood. Called oriented strand board (OSB), the
panels are cross-laminated, like plywood, and bonded together under intense
pressure. Most panels have a nonskid surface on one side. As with a plywood
panel, an OSB panel's strength runs in both directions because of the cross
lamination.

There are some other benefits:

OSB makes better use of natural
resources. It can be manufactured using smaller, faster-growing trees rather
than those from old-growth forests.

It's cheaper than plywood, which is
one of the reasons builders like it.

It can be produced in larger sheets
than plywood.

Unlike some plywood, OSB does not
contain interior voids.

They offer the advantage of consistent
quality from panel to panel.

Building codes recognize the use of
OSB.

APA Panel Ratings

The Engineered
Wood Association (formerly known
as the American Plywood Association or APA) developed a set of performance
standards for engineered wood sheathing. Under the program, panels are rated
for single-layered flooring, exterior siding, and sheathing for floors, roofs and walls. The panels must meet
requirements for dimensional stability, bond adhesion,
and structural strength, among other things.

The APA stamp on the panels identifies
the end use of the panel. It also provides the span ratings for different products.
For example, a product that can be used as subflooring or as sheathing for a
roof may carry the designation 24/16. In
this case, the left-hand number represents the maximum allowable span in inches
between end supports when the product is used as roof sheathing. The right-hand
number is the span rating for subflooring. The stamp also tells if the panel is
designed to withstand the effects of repeated wetting and drying, called
"Exterior." "Exposure
1" panels are more suited to uses that are not exposed to the weather,
such as subflooring. The thickness of the panel is also listed on the stamp.

Installation

Manufacturers will provide the nailing
schedule for their products. For roof sheathing, the panels are generally installed
using 2½-inch common 8d nails. The nails should be placed 6 inches on center
on end supports, and 12 inches on center at intermediate framing. Because OSB
is very dry when it leaves the factory, some swelling can be expected as the panels absorb moisture from
humidity. To account for this, most manufacturers recommend leaving an 1/8-inch
gap between panels.

Standard panels are 4x8 feet. As with
plywood, it is important to support the panel when cutting it to size. On the job site, builders usually cut panels using circular
saws equipped with a plywood/OSB blade. These blades have more and finer teeth
than the typical framing or ripping blade and produce a finer, cleaner cut than
the framing blade.

Engineered wood sheathing’s industrial
durability makes it a standard for building purposes. Use these tips to ensure
a quality finish whenever you’re working with it.

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Fran Donegan writes
home- and garden-related content for numerous digital and print publications.
He is the author of the books “Pools
and Spas” and “Paint Your Home.”
To see a wide selection of the kinds of
circular saws and blades needed for projects like these, check out the Home Depot
website.